While in general I agree with your sentiment, I thought about this question before and one possible answer I came up with was to save the wear and tear on the 2nd drive. i.e. it only uses the 2nd drive when the 1st one have too much ECC.

This approach matches well with the raid 1 goal of ultimate redundancy.

Ultimately, I wish more controller would expose the finer details on Raid tuning such as this optionReply

I see your point that the reads won't be 100% sequential as seen by the drive heads, but if drive 1 starts reading at X and drive 2 at X+128KB, you can effectively get twice the read throughput over 256KB. Then you have to move the drive heads +128KB which does incur a performance cost.

Still with a sufficiently large read block size, I would think there could be a substantial performance improvement reading from both drives in RAID 1. Does anyone know a RAID1 HW or SW controller that can do this?Reply

The time spent skipping ahead is equal to the the time spend reading the area being skipped in a non-fragmented file. To double read speeds in a "mirrored" drive you'd need to have either the array controller or the driver in a software array store the file sectors as 02481357... on the first drive and 13570248... so that when reading the file the two drives are reading sequential sectors on the drive and alternating chunks of data in the file.Reply

No, you wouldn't. You'd just need to alternate drives for reads, keeping them balanced, so that a total QD of say, 6 would be QD=2-4 on one drive, and QD=2-4 on the other. Where the file data actually gets stored shouldn't matter, only how the RAID implementation decides to read it. If the reads are sufficiently sequential, both drives should be able to stay quite busy, and get read performance around that of RAID 0.

Most likely is that they didn't bother even trying that, as RAID 1 is not generally used for performance anyway.Reply

This is a good question. I assume that most drives are set up in their controllers to present data sequentially from the beginning. I don't think it's very common that any type of program would ask a storage device for the second-half of a given file (at least not without having read the first half); I would think that the drive wouldn't have the capability within itself to address data beginning at an arbitrary point in a sequence of data - it always has to start at the beginning of the data(?).

I think to implement this you would need to segment your data at the storage/RAID controller level, like striping but each drive has all the stripes in a RAID 1. Then at the controller level the controller would be able to take a request for data and, assuming the requested data spans at least two segments, it can produce two or more starting-addresses for the drives to read. But then your segment-size would have to be tuned to the kind of data you have (like allocation units) and also then there would be an additional level of addressing abstraction/complexity that would make any kind of data-recovery very difficult.

Everything I just said may be wrong. I'm just making assumptions and inferences because it's fun. Let's get a volunteer who has more knowledge or feels like trawling wikipedia for a while!Reply

Yes, I'm thinking this would be best done at the controller level. I've seen operating systems apply their own striping of sorts at the filesystem (i.e. NTFS) level. Try writing two large files simultaneously to the same hard drive. On an OS like Windows 8, the throughput is surprisingly good. This can only be achieved if the OS is smart enough to use a reasonably large "chunk" size for writing the file fragments to the disk. In this way the disk sees mostly sequential write activity despite the two concurrent write operations, while the number of file fragments tracked by the filesystem is minimized.Reply

If you put it that way, then $100 for the enclosure, PSU and controller board would be reasonable, so it is a good buy if a DAS suits your needs using USB3 only interfaces with the added value of a hub tossed in as extra!.Reply

Um, OK. Is there any reason why it can't connect to your router or Plex server? While the review is a little ambiguous, there's no mention of needing added OS-specific drivers just to see the drives, so it *may* work with [most USB UMC enabled] routers just fine.Reply

What problem do you want to solve on a Mac?This will give you a single glob of 8GB storage with minimal config, but you're paying for that convenience. That's fine, but there are cheaper and/or higher performing alternatives.

If you're willing to do just a little config, for the same sort of price you could buy- a USB3 hub- a 256GB external USB3 SSD- two USB3 4TB hard drivesYou could then use Apple SW RAID to stripe the HDs together, and use CoreStorage (using the commandline diskutil command) to fuse the SSD to the striped RAID. What you'd have will give you the performance of this box for throughput, but with the zippiness of SSDs for the random access. I have a system like this (although put together from substantially older equipment --- an old 64GB SSD and two REALLY old 300GB HDs) and it works astonishingly well given the age of the equipment, especially the HDs. Reply

AT authors work remotely (and live all over the world) so there isn't a single shared testbed, nor can they easily loan hardware back and forth for testing. Since Apple doesn't donate hardware to build testbeds, the only authors who have Apple devices to test with are those who've bought Apple computers with their own money for personal use.Reply

If the layout (stride and chunk size) is optimal, it would be close to RAID-0 for multithreaded workloads, since both disks can be read independently. Actually even single threaded/single-process workloads can benefit if the program is using asynch/queued IO.... but this *IF* the DAS unit were using linux (mdadm) or software raid.

However it's not, it's using a very simple Jmicron controller to do the mirroring. Reply

I've never before heard that a raid1 component drive might not be universally readable. All the controller is supposed to do is make the two drives identical. A single component of the RAID should be readable by any other system that supports that filesystem. Reply

Would this, or something similar from another manufacturer, work connection to a USB 2.0 port? I would like to use this connected to an older machine, but I don't really want to add a USB 3.0 adapter card. If the device slowed to USB 2.0 speeds that would be fine. Reply

" Inserting the removed disk into a PC's SATA slot didn't show the stored data (as expected, since this is hardware RAID)."

This defies all sense. RAID is RAID whether it is software or hardware. Maybe the reason the drive wasn't readable is because it was a RAID 0 component? That would never be readable as such under any circumstances. But a RAID 1 drive should always be readable.